Conventionally, US 20060169251 (JP-A-2006-207499) discloses a fuel feed apparatus provided to an accumulator fuel injection system of a diesel engine. The fuel feed apparatus includes a feed pump and a high-pressure pump. The feed pump supplies fuel from a fuel tank to the high-pressure pump. The high-pressure pump press-feeds high-pressure fuel to a common rail, which is provided for accumulating the high-pressure fuel in the accumulator fuel injection system.
The fuel feed apparatus in the US 20060169251 further includes a fuel filter and a relief valve. The fuel filter is provided downstream of the feed pump for filtering fuel. The relief valve releases fuel, which applies pressure to the fuel filter, toward the fuel tank when the pressure becomes equal to or greater than predetermined pressure.
In this structure, the fuel filter can be restricted from being plugged by applying pressure of the feed pump to the fuel filter, thereby sufficiently supplying fuel to the common rail, even when viscosity of fuel increases in, for example, a low temperature condition. In addition, the relief valve restricts the fuel filter from being applied with excessive pressure.
In the fuel feed apparatus disclosed in the US 20060169251, the feed pump press-feeds fuel, and the fuel returns to the fuel rank when the relief valve opens. In this structure, inlet flow of the feed pump needs to be increased, at least when the relief valve opens, to properly supply fuel to the high-pressure pump.
However, when the inlet flow of the feed pump is increased, pressure loss caused between the fuel pump and the feed pump becomes large, and consequently, suction load of the feed pump increases. Such increase in suction load may cause abnormal ablation in components of the feed pump, and may shorten a lifetime of the feed pump.
It is conceivable to increase the diameter of the inlet pipe between the fuel tank and the feed pump to decrease the pressure loss. However increase in the diameter of the inlet pipe enlarges the fuel feed apparatus.